The present invention relates to a method for the elimination of the d.c. voltage components of a capacitive a.c. voltage divider, especially for middle and high voltages, as well as to a circuit for carrying out this method and to an apparatus with such a circuit.
Such a method, respectively, such an arrangement are described already in the DE-OS No. 26 34 595. The series circuit of a switch with a very low ohmic resistance is connected in this prior art arrangement as switching device in parallel to the condenser of a capacitive voltage divider connected to ground, and additionally a control device is connected in parallel thereto. The control device opens the switch after the reconnection of the a.c. voltage in dependence of the first passage through zero after a delay time of about one millisecond. Under normal operating conditions, the normal operating condition is reached thereby no later than a half cycle after the reconnection to the line and the input is short-circuited during this period of time.
However, the measurement of high voltages takes place at present for the most part by means of inductive voltage measuring transformers. The higher the voltage level, the more expensive become these installations.
Consequently, attempts were made in the past every so often to carry out the high voltage measurement by means of capacitive high voltage dividers and following measurement amplifier.
The occurrence of d.c. voltage terms in the capacitive divider thereby represents a problem. These d.c. voltages occur when the divider which is under high voltage, is disconnected at an instant in which the voltage is not equal to zero. A d.c. voltage then remains at the amplifier input. Even if this d.c. voltage is reduced by way of the input resistance of the amplifier, the charge which is stored in the high-end voltage condenser, occurs as d.c. voltage component during the reconnection of the high voltage, which d.c. voltage component is subadjacent to the a.c. voltage. This displaces the zero passages of the output voltage which are important for protective installations and with potential-free or floating transformer outputs leads to the saturation thereof.
In improving the aforementioned arrangement, a circuit is proposed in the DE-OS No. 28 46 285 in which the discharge impedance is interconnected in case of a determination of d.c. voltage components and is again disconnected after some cycles which causes the disadvantages described above only during a shorter period of time.
A second high voltage divider is provided in this literature which feeds a second amplifier input blocking a d.c. voltage, and dividers were proposed which include several measuring or control condensers in order to differently process different input voltages and to make logic decisions which alternately connect and disconnect for several half cycles an element of variable impedance at the measurement input in the sense of the d.c. voltage until the d.c. voltage has decayed.
The element with variable impedance is in the most simple case a resistance, and a switch arranged in series therewith. However, it may also be a semi-conductor controllable in its resistance or a register of switches with the same or different resistances as are known in connection with digital-analog converters.
Furthermore, amplifiers with purely electronic filters were made which filter the d.c. voltage components out of the measurement signal. However, it is thereby very difficult to bring about an acceptable compromise in the present-day requirements for accuracy in amplitude and phase, in the frequency response and the transient behavior. Furthermore, such an arrangement is difficult to monitor for its operating reliability--as takes place customarily with the so-called longitudinal differentiation--because the output voltage is not equal to the input voltage for a transition period of time.
Present-day rapid protective installations require already after milliseconds amplifier output voltages which are a good image of the high voltage and which contain no shifts or displacements leading to error or faulty switchings.
This task leads to the solutions known in the prior art according to the DE-OS No. 28 46 285 and the DE-OS No. 26 34 595. The basic concept of the DE-OS No. 26 34 595 is--in contrast to the subject matter according to the DE-OS No. 28 46 285--that the d.c. voltage component is not removed gradually in several steps but is removed totally from the simple high voltage divider in a short period of time and instantaneously at the earliest possible instant.
A falsification of the a.c. voltage to be measured always occurs with these prior art short-circuit arrangements in the middle and high voltage range.
In contrast thereto, the underlying problems are to be solved by the present invention to remove the d.c. voltage component of the a.c. measuring voltage as rapidly as possible and to the greatest possible extent without falsification of the reproduction of the middle or high voltage to be measured. Large, costly components such as, for example, inductive voltage converters, short-circuit switches or the like are thereby to be avoided.
The underlying problems are solved according to the present invention in that the a.c. measuring voltage is fed to a measuring amplifier by way of a condenser and by way of two parallel lines whereby the a.c. measuring voltage is fed directly by way of one line and the d.c. voltage component of the a.c. measuring voltage is determined in the other line in that the a.c. measuring voltage is divided into such small time section that an equal number of time sections is coordinated to each cycle or half cycle and the a.c. measuring voltage is continuously integrated over one cycle at the frequency of the time sections and the thus-determined respective d.c. voltage value is fed in phase opposition to the original value to the measuring amplifier and by way of the latter to the measurement transformer.
By digitalizing the a.c. measuring voltage in a parallel branch of the amplifier and by the integration of the d.c. voltage component obtained over a cycle of the a.c. measurement voltage and the inverted input thereof into the measurement amplifier, any influence on the a.c. measurement voltage is completely avoided and additionally the d.c. voltage component is completely compensated in a very short period of time. By the integration over a cycle, additionally higher frequency interference or noise voltages are eliminated so that actually only the pure d.c. voltage component is obtained and an accurate compensation is made possible thereby.
The same end result is obtained if the a.c. measuring voltage is digitalized in a parallel branch and forms the average value from several time sections of a half cycle and compares the same with the average value formed a half cycle earlier and feeds the obtained voltage difference inverted to the measuring amplifier. One obtains thereby a compensation already at an earlier point in time.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, two embodiments in accordance with the present invention, and wherein: